1. Field of the Invention
This invention relates to a charging system for portable electronic equipment, specifically to a charging system that charges a rechargeable battery (secondary battery) incorporated in the portable electronic equipment through a USB (Universal Serial Bus) interface.
2. Description of the Related Art
The USB is an industry standard of a universal serial bus to connect various kinds of peripheral devices to a host device. In recent years, a USB battery charger using the USB has come into use in order to charge a battery incorporated in portable electronic equipment such as a mobile phone. When the portable electronic equipment is connected to the USB battery charger through a USB cable, the battery incorporated in the portable electronic equipment is charged through a VBUS terminal (power supply terminal) in a USB connector.
In addition to a conventional USB hub, there have appeared dedicated battery chargers (indoor on-wall type battery charger, in-car battery charger, etc.), accessory charger adapters and USB battery chargers for tablets.
There are various types of USB battery chargers that differ from each other in charging current providing capacity (0.1 A, 0.5 A or 2.1 A, for example). Therefore, the portable electronic equipment identifies the type of the battery charger by monitoring voltages at data terminals D+ and D− in the USB connector and limits an amount of charging current to take-in by a current limiting circuit in accordance with the charging current providing capacity. If the portable electronic equipment takes in the charging current exceeding the charging current providing capacity of the USB battery charger, a voltage at the VBUS terminal is reduced because of a reduction in an input impedance of the portable electronic equipment. Thus, the USB battery charger stops providing the charging current as specified by the USB standard so as to prevent destruction of the device or the like.
The charging system using the USB battery charger is disclosed in Japanese Patent Application Publication No. 2011-223669, for example.
In some cases, however, monitoring the voltages at the data terminals D+ and D− is performed incorrectly due to influence of chattering noise caused when the portable electronic equipment is connected to the USB battery charger. As a result, there is a possibility that the type of the USB battery charger would be misidentified and the amount of charging current taken into the portable electronic equipment would exceed the charging current providing capacity of the USB battery charger. Also, there is a possibility that the current limiting circuit would malfunction due to influence of other kind of noise and the amount of charging current taken into the portable electronic equipment would exceed the charging current providing capacity of the battery charger. The USB battery charger stops providing the charging current when the amount of charging current taken into the portable electronic equipment is not limited appropriately. That results in discontinued charging of the battery, which is very inconvenient for the user of the portable electronic equipment.
Considering the above, this invention is directed to automatically restarting providing the charging current in the case where providing the charging current from the USB battery charger is stopped.